Several patents disclose the preparation of modified diisocyanates which contain allophanate linkages. See for instance British Patent 994, 890 which discloses the reaction of diisocyanates with glycols and triols to form urethane polyisocyanates which are further reacted in the presence of heat and a metal catalyst to provide allophanate polyisocyanates. U.S. Pat. No. 4,738,991 teaches that the reaction of a molar excess of monomeric diisocyanates with polyhydric alcohols, which include both aliphatic and aromatic compounds such as ethylene glycol, trimethylene glycol, 1,4-butanediol, bisphenol A, thereof, in presence of certain specified catalysts produces polyisocyanates characterized by allophanate linkages. U.S. Pat. No. 5,319,053, U.S. Pat. No. 5,319,054, U.S. Pat. No. 5,44,003 teach that modified liquid diphenyl diisocyanate containing allophanate linkages can be synthesized from an aliphatic alcohol and monomeric diphenylmethane diisocyanate in presence of a catalyst. None of these patents disclose modified polymeric aromatic isocyanates containing allophanate linkages.
One of the major processes used in the foundry industry for making metal parts is sand casting. In sand casting, disposable foundry shapes (usually characterized as molds and cores) are made by shaping and curing a foundry mix which is a mixture of sand and an organic or inorganic binder. The binder is used to strengthen the molds and cores.
Two of the major processes used in sand casting for making molds and cores are the no-bake process and the cold-box process. In the no-bake process, a liquid curing agent is mixed with an aggregate and shaped to produce a cured mold and/or core. In the cold-box process, a gaseous curing agent is passed through a compacted shaped mix to produce a cured mold and/or core. Polyurethane-forming binders, cured with a gaseous tertiary amine catalyst, are often used in the cold-box process to hold shaped foundry aggregate together as a mold or core. See for example U.S. Pat. No. 3,409,579. The polyurethane-forming binder system usually consists of a phenolic resin component and polyisocyanate component which are mixed with sand prior to compacting and curing to form a foundry mix. None of the patents, previously discussed, which relate to modified diisocyanates containing allophanate linkages, suggest the use of such modified diisocyanates in foundry applications